As it is preferred to increase the cross sectional area of the passage leading to a combustion chamber in order to improve the volumetric efficiency of the engine, it has become increasingly common to provide a plurality of intake valves and/or exhaust valves for each cylinder with the aim of maximizing the effective area of the valves in relation with the internal surface area of the combustion chamber.
Also is known the valve control technology known as combination valve timing according to which a swirl is produced in the mixture introduced into the combustion chamber by shifting the opening and closing timing of the plural valves (Japanese patent laid-open publication No. 59-147822).
The temperature of combustion gas is extremely high and its flow speed may reach the sonic speed. It is therefore desirable to minimize the flow resistance of the exhaust passages to improve exhaust efficiency by taking advantage of the flow speed of exhaust gas.
The exhaust manifold which merges the exhaust passages leading to the exhaust ports opening out into the cylinder head is generally made of cast iron because of the heat resisting property of the material, and the exhaust manifold is desired to be made as small as possible to make room for mounting accessory equipment and to reduce the overall weight of the engine. A similar consideration applies also to the intake system of the engine.